Synthesis, Structure, and Thermal Properties of Volatile Group 11 Triazenides as Potential Precursors for Vapor Deposition

Group 11 thin films are desirable as interconnects in microelectronics. Although many M–N-bonded Cu precursors have been explored for vapor deposition, there is currently a lack of suitable Ag and Au derivatives. Herein, we present monovalent Cu, Ag, and Au 1,3-di-tert-butyltriazenides that have potential for use in vapor deposition. Their thermal stability and volatility rival that of current state-of-the-art group 11 precursors with bidentate M–N-bonded ligands. Solution-state thermolysis of these triazenides yielded polycrystalline films of elemental Cu, Ag, and Au. The compounds are therefore highly promising as single-source precursors for vapor deposition of coinage metal films.


■ INTRODUCTION
Thin films of group 11 metals are highly desirable as interconnects in integrated circuits due to their excellent electrical and thermal conductivity and resistance to electron migration. 1 Furthermore, transparent Ag thin-film electrodes have potential for solar cell applications, 2 while Au is advantageous for chemical and biological sensors. 3 Today, Cu, Ag, and Au films are commonly deposited by vapor deposition techniques. 4,5 Chemical vapor deposition (CVD) and atomic layer deposition (ALD) are two methods currently used to deposit high-quality thin films of group 11 metals. To be successful, both methods require precursors that are sufficiently volatile and thermally stable for transport from the source to the reaction chamber without decomposing. In CVD, the precursors are mixed in the reaction chamber and react, both in the gas phase and on surfaces, to deposit the target material. In ALD, the precursors are added to the system sequentially to allow the process to be governed by selflimiting surface reactions. To date, there are more precursors known for Cu compared to Ag and Au, and thus fewer deposition processes are reported for the latter metals.
The amidinate and guanidinate ligand systems (Figure 1a and 1b, respectively) have been used to produce volatile and thermally stable transition-metal precursors for vapor deposition. 6−9 A drawback of these precursors is their tendency to decompose via two pathways: β-hydride elimination and carbodiimide (CDI) deinsertion. 10,11 While β-hydride elimination is easily blocked by having exocyclic N-substituents free from β-hydrogens, suppressing CDI deinsertion is more difficult as it involves the endocyclic-carbon substituent. Metallic Cu films have been deposited by ALD using Cu(I) amidinates 6,12−19 and guanidinates. 20−22 The Ag(I) and Au(I) amidinates and guanidinates are thermally unstable with respect to CDI deinsertion and therefore have not been successfully used for vapor deposition. 11,23 A similar ligand to the previous two is the iso-ureate (Figure 1c), which has yielded Cu(I) precursors for CVD of metallic Cu films. 24 The thermal behavior of these compounds was similar to that of Cu(I) guanidinates and hypothesized to undergo thermolysis via the same mechanism. 21 The iminopyrrolidinates are monocyclic amidinates, where the R 2 substituent is bound to the nitrogen forming a pyrrolidine ring (Figure 1d). 10 Tethered to a nitrogen in the ligand backbone, the R 2 substituent is difficult to access for CDI deinsertion, making the iminopyrrolidinates more thermally stable than the acyclic amidinates. 11 Thus, not only have the monovalent group 11 iminopyrrolidinates afforded Cu films by ALD 25 but also Ag and Au films, with ∼3 at. % carbon, by low-temperature CVD. 5 Further constrained, bicyclic amidinates ( Figure 1e) have been used to yield monovalent group 11 compounds with improved thermal stability over the iminopyrrolidinates. 26,27 Using these compounds with H 2 afforded Ag and Au films with ∼6−7 at. % carbon by low-temperature CVD. 27 An alternative approach to develop new M−N-bonded precursors is to alter the N−C−N ligand backbone. The triazenides differ from the amidinates by having a nitrogen atom in place of the endocyclic carbon ( Figure 1f). Replacing the tetravalent carbon with a trivalent nitrogen effectively removes the R 2 substituent involved in CDI deinsertion and effectively blocking this decomposition pathway. Monovalent group 11 triazenides exist in the literature and all except one possess 1,3-diaryltriazenide ligands. 28−33 These 1,3-diaryltriazenide examples, however, are most likely not volatile due to their increased intermolecular interactions (e.g., πstacking) and therefore rendering them unsuitable for vapor deposition. The only 1,3-dialkyl analogue in the literature is a tetranuclear Cu(I) compound with 1,3-dimethyltriazenide ligands, where only melting point (185−186°C) and structural data have been discussed. 34,35 Recently, we reported the first examples of volatile group 13 and 14 dialkyltriazenides. 36−40 The Ga and In triazenides have been used as ALD precursors to afford excellent-quality GaN, InN, InGaN, and In 2 O 3 . 36,37,41−43 With the success of the triazenide ligand to produce volatile and thermally stable group 13 and 14 compounds, we decided to investigate its reactivity with monovalent coinage metals. Herein, we report the synthesis, structure, and thermal properties of monovalent group 11 triazenides. Their ease to produce, high volatility, and thermal stability make these new precursors highly interesting for use in vapor deposition.

■ RESULTS AND DISCUSSION
Compounds 1, 2, and 3a were obtained in good yields by reacting MCl (M = Cu, Ag, Au) with lithium 1,3-di-tertbutyltriazenide 39 in tetrahydrofuran (THF) followed by recrystallization (Scheme 1). The compounds were fully characterized by nuclear magnetic resonance (NMR), elemental analysis, melting or decomposition points, and Xray crystallography. Crystals of 1−3a did not degrade when stored for weeks under ambient conditions. When immersed in water for 2 weeks, 1 showed slight green discoloration, while 2 and 3a remained unchanged.
The 1 H NMR spectra of 1 and 3a in C 6 D 6 showed one singlet (1.27 ppm) suggesting exclusively dinuclear species in solution state. Variable-temperature (VT) NMR showed no line splitting for 1 between −20 and 70°C. Compound 2 showed two mildly broadened singlets (1.27 and 1.43 ppm) indicative of an equilibrium between di-and tetranuclear forms. Similar di-/tetranuclear and di-/trinuclear equilibria were previously found for Ag formamidinate 44 and Ag acetamidinate 7 compounds, respectively. Varying the concentration of 2 changed the di-/tetranuclear ratio. As expected, the relative concentration of the tetranuclear species increased for more concentrated samples of 2 (Figure 2a). Using a coordinating solvent (THF-d 8 ) did not affect the equilibrium (see the Supporting Information). Using the data, the ambient temperature dissociation constant of 2, K diss , was estimated to be 28.7 ± 0.4 mM. VT NMR on a 3.8 mM sample of 2 showed that the equilibrium shifted toward the dinuclear form between 30 and 60°C (Figure 2b). A van't Hoff plot of ln[K diss (T)] versus T −1 gave ΔH and ΔS of dissociation values of +38.6 kJ mol −1 and 98.9 J K −1 mol −1 , respectively (see the Supporting Information).
An additional signal emerged at 1.53 ppm for the NMR sample of 3a when stored at room temperature for months. The new spectra showed two signals, like that of 2, suggesting a presence of di-and tetranuclear forms of the Au triazenide. No change was observed in the di-/tetranuclear ratio for 1 H NMR spectra acquired between 5 and 60°C. The dinuclear form appears to be metastable, and the dinuclear to tetranuclear transformation is slow and, most likely, irreversible. In fact, a pure tetranuclear form, compound 3b, was obtained by heating a solution of 3a in toluene to 150°C for 3 days followed by recrystallization (Scheme 2). Compound 3b was fully characterized as per compounds 1−3a.
Diffusion-ordered spectroscopy showed faster diffusion for 1 and 3a compared to 3b, which is consistent with 1 and 3a being dinuclear and 3b being tetranuclear. Similar results were found for 2, where the 1.27 ppm species gave faster diffusion than the 1.43 ppm species. However, the 1.27 ppm species showed slower diffusion than 1 and 3a, while the 1.43 ppm species showed faster diffusion than 3b, likely an artifact caused by overlap in the chemical shift dimension of the signals in 2.
X-ray crystallography of 1 and 3a showed dinuclear structures with two bridging 1,3-di-tert-butyltriazenide ligands on opposite sides of the metal centers forming a planar metallacycle ( Figure 3). All M−N and N−N bond lengths are equivalent, indicating that the electrons are delocalized over the M and N centers. The structures are consistent with their respective 1,3-bis(2,6-diisopropylphenyltriazenide) (dipp 2 N 3 ) analogues, which has the aromatic rings noncoplanar to the metallacycle (Table 1).
Both 2 and 3b showed tetranuclear molecules with the metal centers in rhombus conformations. Four 1,3-di-tert-butyltriazenide ligands bridge the metal centers along the perimeter, alternating above and below the plane (Figure 4a). The structures gave similar bond parameters for all but their diagonal M···M distance, and N−M−N and M···M···M angles ( Table 2). Most notably, 2 measured a significantly shorter diagonal M···M distance than 3b. In fact, the diagonal M···M of 2 is only 0.1 Å longer than the average edge M···M, while it is 0.3 Å longer for 3b. This was accompanied by 2 showing M···M···M angles deviating more from 90°than 3b. The structures of 2 and 3b gave similar bond parameters to their respective phenyl analogues, the 4-fluorophenyltriazenide (4Fdpt) Ag (the only other known tetranuclear Ag triazenide in the literature) and dpt Au. 29,33 However, both 2 and 3b gave more acute M···M···M angles and therefore shorter diagonal M···M distances.
Prior to the first structure-determination attempt of 3a, the crystals were stored for weeks with the residual mother liquor from the recrystallization. As opposed to the dinuclear structure of 3a, however, a tetranuclear molecule was observed with the Au atoms in a buckled square conformation ( Figure  4b). The residual mother liquor must have facilitated the reaction of 3a into the tetranuclear form, which then crystallized in the buckled square conformation (structure 3c). Structure 3c was only observed by X-ray crystallography and differ from 3b only by adopting a different conformation in the solid state. Thus, any discussion of structure 3c is limited to the obtained crystal structure.
Structure 3c showed a significant difference in N−N lengths for two opposing ligands (0.108 and 0.066 Å). A similar, albeit smaller, difference was found in the Au diphenylacetamidinate, which also adopted the buckled square conformation. 45 The ligands in structure 3c are twisted relative to the metal centers they bridge, while 3b showed only minor twisting (N−M···M− N: ∼18 and 2.6°, respectively). Similar buckled square conformation is found in Cu and Ag compounds employing triazenide and bicyclic guanidinate-like ligands, respectively, and are attributed to steric effects. 31,46 Although serendipitous, it is unclear exactly why the buckled square conformation 3c was obtained, rather than rhombic seen for 3b. It may be solvent-dependent; however, the tetranuclear species most likely converts rapidly between the buckled square and rhombic conformation in solution state, which is supported by 1 H NMR of 2 and 3b showing only one Scheme 2. Transformation of 3a into 3b Figure 3. ORTEP drawing of 1. Thermal ellipsoids at the 50% probability level. All hydrogen atoms were omitted for clarity. Compound 3a gave an analogous structure molecular geometry analogous dinuclear structure (see the Supporting Information).

Inorganic Chemistry pubs.acs.org/IC
Article signal for the tetranuclear species at 25°C. We made no attempts to replicate these results. DFT and NBO Calculations. The DFT geometries of 1−3 are in good agreement with their respective crystal structures (see the Supporting Information). Singlet electron configurations gave far better agreement than triplet configurations. Rhombic 2 showed the largest deviation, where the optimized geometry gave Ag···Ag···Ag angles closer to 90°and, consequently, a longer diagonal Ag···Ag distance compared to the crystal structure (see the Supporting Information).
Among the functionals used, LC-ωHPBE yielded a geometry with the best match for rhombic 2 and was therefore used for all calculations. Structures were also optimized for dinuclear 2 and 3 and rhombic 1 and 3 to calculate the electronic energy difference (ΔE 0 ) between the two different structures (eq 1).
Preference for the tetranuclear form increases in the order 1 < 2 < 3 where 1 slightly favored the dinuclear form, while 2 and 3 favored the rhombic ( Table 3). The ΔE 0 was negligible   Due to the decrease in entropy upon combining two dinuclear species into a tetranuclear, the difference in Gibbs free energy (ΔG) for 1−3 (calculated analogously to ΔE 0 ) favored the dinuclear structures in the gas phase and increased in the order 3 < 2 < 1. Thus, if 1−3 form di-/tetranuclear equilibria, the dinuclear structures are expected to dominate in the gas phase. However, while 2 showed an equilibrium in C 6 D 6 by NMR, 3 did not and was even isolated in its di-and tetranuclear forms (3a and 3b, respectively).
The optimized structures of 1−3 were investigated by natural bond orbital (NBO) analysis. Natural bond orders for dinuclear 1−3 from natural resonance theory calculations agree with the expected formal bond orders. The M−N bonds showed high ionic character, ∼100, 84, and 74% for dinuclear 1, 2, and 3, respectively. Wiberg bond indices (WBIs) showed significantly greater M−N bond order for the Au structures 3 than that for 1 and 2 ( Table 4). The greater M−N WBI for 3 is consistent with NMR experiments of 2 showing a di-/ tetranuclear equilibrium while 3 does not. WBI suggests small M···M interactions for 1−3, ranging between 0.04 and 0.06 (see the Supporting Information).
The metal center natural charges for 1−3 decreased in the order 1 > 2 ≫ 3 (Table 5), as expected based on electron affinities for Cu−Au. Rhombic 1 gave ∼0.02 au greater metal center charge than the dinuclear structure. In contrast, dinuclear 2 showed greater charges than rhombic 2 but with a smaller difference (∼0.01 au), while 3 showed similar metal center charges between di-and tetranuclear structures (difference < 0.005 au).
Second-order perturbation theory analysis (donor−acceptor interactions) from the NBO analysis was used to compare stabilization energies (E (2) ) for donor−acceptor interactions of rhombic 1−3 with two of their respective dinuclear structures (eq 2).
Interactions involving valence acceptor orbitals stabilized dinuclear 1−3 more than their respective tetranuclear forms (ΔE (2) < 0 kcal mol −1 ) due to more favorable orbital overlap. For example, geminal n1 N → n M * (which is essentially σ MN ) greatly favored dinuclear 1−3 over rhombic, and ΔE (2) decreased in the order 3 < 2 < 1 ( Table 6). In general, the dinuclear forms were favored for interactions involving valence acceptors where the number of interactions remained unchanged between two dinuclear and a rhombic structure (e.g., geminal M−L). The geminal M···M interactions, however, favored the rhombic structures as they compensated for their lower E (2) by having more interactions.
Interactions involving Rydberg acceptor orbitals often favored rhombic 1−3 over dinuclear (ΔE (2) > 0 kcal mol −1 ) due to similar orbital overlap or sometimes even larger overlap for the rhombic structures. Vicinal and remote interactions favored the rhombic structures due to their larger number of interactions over two dinuclear. In contrast to 2 and 3, structures of 1 interacted poorly with Rydberg acceptors and gave modest ΔE (2) . Therefore, rhombic 1 was not stabilized sufficiently to compensate for the destabilization relative to the dinuclear form (e.g., n1 N → n M *), while Rydberg acceptors had larger impact on rhombic 2 and 3. This may explain why the crystal structure of 1 showed a dinuclear structure, while 2 and 3 were tetranuclear. See the Supporting Information for more details on the NBO analysis.
To summarize, in the context of second-order perturbation theory, the rhombic structures must adapt and retain orbital overlap from the dinuclear structure, particularly for the geminal M−N. Rhombic 1−3 showed more M···M interactions than two of their respective dinuclear structures. Strong M···M interactions therefore favor one rhombic structure over two dinuclear. For the same reason, strong vicinal and remote interactions favor the rhombic structures. Last, if accessible, Rydberg acceptor levels are highly impactful, favoring the rhombic structures by having comparable orbital overlap between the dinuclear and rhombic forms.
Thermal Analysis. Compounds 1, 2, and 3a sublimed between 120 and 130°C at 0.5 mbar, while 3b sublimed between 140 and 160°C, all quantitatively. The similar sublimation temperatures suggest that 1, 2, and 3a volatilize in their dinuclear forms. Additionally, 3a and 3b remained unchanged by NMR after sublimation which suggests that these sublime in dinuclear and tetranuclear forms, respectively. Thermogravimetric analysis (TGA) ramp experiments showed one-step volatilization for 1, 2, and 3b ( Figure 5) where 1 and 3b gave the lowest and highest onset, respectively (Table 7). Compound 2 showed considerably lower onset than 3b which supports the suggestion that the tetranuclear 2 first forms dinuclear species that then volatilize. A similar two-step process was postulated for the bicyclic Ag amidinate to explain why its volatilization kinetics resembled that of the dinuclear bicyclic Cu amidinate. 27 Compound 3a showed two mass loss events. The first and second steps coincide with that of 2 and 3b, respectively. Most likely, dinuclear molecules are volatilized in the first step. This volatilization is disrupted at ∼160−200°C to give the first slanting plateau as 3a is depleted to form 3b. As the temperature increases further, 3b gains sufficient heat to volatilize, leading to the second mass-loss event. Compound 1 gave a residual mass <1%. In contrast, 2 gave 7% residual mass, suggesting a small degree of thermolysis at The residual masses of 3a and 3b are lower than the mass % Au in their empirical formula (∼56%), indicating that there is some volatilization of Au species. The outlet gas of the TGA was monitored using a mass spectrometer scanning for m/z 57 and 99 (matching tBu + and tBuN 3 + , respectively). In all cases, mass-loss steps gave an increased signal for m/z 57. Additionally, 2 and 3a showed a strong and weak signal, respectively, for m/z 99. For 3a, the second step showed significantly stronger signals for both m/z 57 and 99 than the first step (see the Supporting Information). It is likely that detection of tBuN 3 + occurs during thermolysis of the compounds. Comparing TGA ramp experiments performed under similar conditions showed that 1−3 are more volatile than their corresponding mono-and bicyclic amidinates. 26,47 The Ag triazenide 2 gave a residual mass similar to the cyclic Ag amidinates (>10% for 10 mg samples). 5,27 The Au compounds 3a and b gave a comparable residual mass to the cyclic Au amidinates (∼40% for 3a and 3b, compared to 40 and 25% for the mono-and bicyclic Au amidinate, respectively). 5,27 Solutions of 1−3a in toluene-d 8 were heated between 100 and 230°C in flame-sealed heavy-walled NMR tubes and monitored periodically by 1 H NMR. The change in integrals was tracked using the residual solvent as reference (see the Supporting Information). Compound 1 showed minor changes below 180°C. The tube walls turned slightly yellow above 140°C . This discoloration became more pronounced at 190°C, and additional signals appeared in the base line by NMR. Above 190°C, these signals became stronger as the signal of 1 diminished more rapidly. However, 1 was still seen after 3 h at 230°C. Compound 2 started decomposing at 120°C, and a metallic coating formed on the tube walls. The signals of 2 diminished steadily between 130 and 160°C to give a few singlets, a broad feature at 1.0−1.3 ppm and a multiplet at 4.69−4.72 ppm. Much of 2 remained at 160°C; however, lock issues prevented further NMR experiments. Compound 3a converted into 3b already at 100°C (singlet at 1.49 ppm). The solution was held at 120°C for ∼48 h which transformed most of 3a into 3b. A dark brown or black precipitate formed during the transformation; however, only traces of impurity signals appeared as 3b formed. A golden metallic film was observed on the tube walls at 140°C. Upon further heating, the trace of 3a slowly diminished. Meanwhile, 3b remained steady below 180°C and then diminished rapidly between 180 and 200°C to give several high-intensity singlets. These new signals matched those observed for the thermolysis of 1. After complete thermolysis of 1, 2, and 3a, metallic films had deposited on the interior of the NMR tubes (see the Supporting Information). X-ray powder diffraction revealed the films to be polycrystalline elemental Cu, Ag, and Au ( Figure 6). The ability of these precursors to form elemental films upon thermolysis makes them promising candidates as single-source precursors for vapor deposition of coinage metal films. This is substantial as usually a reducing reagent, such as H 2 gas, is required to reduce the metal center of the precursor to ground state. 48 Solid samples of 1, 2, and 3a were flame sealed under vacuum and heated to 120°C. After 7 days, 1 appeared visually unaffected; no solid particles were seen when dissolving in C 6 D 6 , and 1 H NMR showed no byproducts from thermolysis. In contrast, the crystals of 2 and 3a had visibly darkened after a few hours of heating. After 7 days, 2 and 3a had turned black and slightly purple, respectively, yet 2 remained reflective while 3a did not. Both 2 and 3a was partially insoluble in C 6 D 6 , and 1 H NMR showed a few impurity signals. Additionally, for 3a,

■ CONCLUSIONS
In conclusion, we have presented the first example of volatile Cu, Ag, and Au triazenides (1, 2, and 3, respectively). These compounds showed thermal properties on par with the current state-of-the-art ALD and CVD precursors in the same class but were easier to prepare. Compound 1 showed dinuclear structures. Meanwhile, 2 and 3 showed both dinuclear and tetranuclear (rhombic) forms. In solution state, 2 formed a di-/ tetranuclear equilibrium while 3 did not. Compound 3 was isolated in both di-and tetranuclear forms (3a and 3b, respectively). Overtime, 3a transformed into 3b. In solution state, this transformation happened over many weeks at room temperature or days at 120−150°C. In solid state, 3a only transformed at elevated temperature (120°C). The compounds sublimed quantitatively at reduced pressure, seemingly even the Au analogues. Compounds 1, 2, and 3a sublimed between 120 and 130°C at 0.5 mbar, and 3b sublimed between 140 and 160°C. The sublimation temperatures suggest that 1, 2, and 3a sublime in the dinuclear form, while 3b sublimes in the tetranuclear form, which is supported by TGA ramp experiments, DFT calculations, and NBO analysis. Additionally, solution-state thermolysis of 1−3 yielded elemental group 11 films by X-ray powder diffraction, suggesting that the ligand acts as a reducing agent. Thus, 1− 3 may potentially be used as single-source precursors for CVD of elemental Cu, Ag, and Au films. Overall, these compounds provide an easy-to-produce alternative to the current state-ofthe-art amidinates, as potential ALD and CVD precursors. The 1,3-dialkyltriazenide ligand system is easy to derivatize, allowing it to be tuned as desired.
■ EXPERIMENTAL SECTION General Comments. Caution! As catenated nitrogen compounds are known to be associated with explosive hazards, tert-butylazide, lithium (1,3-di-tert-butyltriazenide) and compounds1−3are possible explosive energetic materials. Although we have not experienced any diff iculties or problems in the synthesis, characterization, sublimation, and handling of these compounds, their energetic properties have not been rigorously investigated and are therefore unknown. We therefore highly recommend all appropriate standard safety precautions for handling explosive materials (safety glasses, face shield, blast shield, leather gloves, polymer apron, and ear protection) be always used when working with these compounds. All reactions and manipulations were carried out under a nitrogen atmosphere on a Schlenk line using Schlenk air-free techniques and in a Glovebox-Systemtechnik dry box. All anhydrous solvents were purchased from Sigma-Aldrich and further dried with 4 Å molecular sieves. CuCl (99.995%), AgCl (99.999%), and Me 2 S· AuCl were purchased from Sigma-Aldrich and used without further purification. Lithium 1,3-di-tert-butyltriazenide was synthesized according to the literature procedure. 39 All NMR spectra were measured with an Oxford Varian and Bruker Avance Neo 500 MHz spectrometers. Solvents peaks were used as an internal standard for the 1 H NMR and 13 C{ 1 H} NMR spectra. The decomposition points were determined in melting point tubes sealed under N 2 with a Stuart SMP10 melting point apparatus and are uncorrected. Elemental analysis was performed by Mikroanalytisches Laboratorium Kolbe, Germany.
Synthesis of Dinuclear (1,3-Di-tert-butyltriazenide)copper-(I) 1. A solution of lithium 1,3-di-tert-butyltriazenide (4.95 g, 3.03 mmol) in THF (20 mL) was added to a suspension of CuCl (3.00 g, 3.03 mmol) in THF (80 mL), and the reaction mixture was heated to 80°C for 24 h in a pressure tube. After cooling down to room temperature, the mixture was concentrated under reduced pressure to give a yellow and brown solid residue. The residue was dissolved in nhexane, filtered through a bed of Celite, and concentrated under reduced pressure to give the crude product as a solid. The crude was recrystallized from n-hexane at −35°C to give 1 as a yellow solid (4.82 g, 72%).

Synthesis of Dinuclear (1,3-Di-tert-butyltriazenide)gold(I) 3a.
A solution of lithium 1,3-di-tert-butyltriazenide (0.28 g, 1.73 mmol) in THF (30 mL) was added to an aluminum-foil-wrapped reaction flask containing a solution of (Me 2 S)AuCl (0.51 g, 1.73 mmol) in THF (30 mL) at −78°C. The reaction mixture was stirred at −78°C for 30 min and at room temperature for 16 h and then concentrated under reduced pressure to give a solid residue. The residue was suspended in n-hexane, filtered through a pad of Celite, and concentrated under reduced pressure to give the crude product as a solid. The crude was recrystallized from n-hexane to give 3a as a solid (0.42 g, 70%).   (1.9, 3.8, 5.7, 7.6, 9.5, 11.4, 13.3, 17.0, and 20.8 mM) were prepared by dissolving the required amount of 2 in 1.0 mL of C 6 D 6 . A ∼400 μL aliquot of each solution was transferred to an NMR tube and used for 1 H NMR analysis. The samples were analyzed using 5°C increments from +25 to +60°C (see the Supporting Information).
Diffusion-Ordered Spectroscopy. Diffusion measurements were performed for 1−3b using bipolar double-stimulated echo pulse sequences with longitudinal eddy-current delay (dstebpgp3s in TopSpin). 49 The following pulse sequence parameters were used for 1 and 3a: relaxation delay = 5 s, diffusion time Δ = 50 ms, gradient pulse length δ/2 = 0.700 ms, and eddy-current delay = 5 ms. The same parameters were used for 2 and 3b, with modifications in Δ (75 and 100 ms, respectively) and δ/2 (1.000 and 1.200 ms, respectively). Measurements were performed at z-gradient strengths varied linearly between 1 and 47 G/cm in 16 increments, with 32 scans at each increment. The diffusion coefficients were extracted using Dynamic Center 2.8.1.
X-ray Crystallographic Analysis. Single crystals were obtained by recrystallization from n-hexane at −35°C for 1, 3a, and 3c and from toluene at −35°C for 2 and 3b. The single crystals were used for X-ray diffraction data collection at 163 K for 1, 2, and 3a; 296 K for 3b, and 153 K for 3c on a Bruker D8 SMART Apex-II diffractometer using graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). All data were collected in hemisphere with over 95% completeness to 2θ < 50.05°. The structures were solved by direct methods. The coordinates of metal atoms were determined from the initial solutions and the N and C atoms by subsequent differential Fourier syntheses. All nonhydrogen atoms were refined first in isotropic and then in anisotropic approximation using Bruker SHELXTL software. Selected crystal data are summarized below.  CCDC 2152400 for 1, 2152401 for 2, 2189288 for 3a, 2189289 for 3b (rhombic conformation), and 2152402 for 3b (buckled square conformation) contain supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre.
Thermogravimetric Analysis-Mass Spectrometry. TGA was performed on alumina pans with a TA instrument Discovery TGA 55. The pans were rinsed with ethanol and then heated by a propane torch until white hot. All TGA experiments were performed under a flow of ultrapure argon (99.999%, 100 sccm). Samples were heated to 500°C at a rate of 10°C min −1 . The onset of volatilization was defined as the intersection between the tangent line of the plateau and slope. The TGA was connected to a Pfeiffer ThermoStar mass spectrometer.
Solution-State Thermolysis. Solutions of 1−3a in toluene-d 8 (40 g L −1 for 1 and 3a, and 20 g L −1 for 2) were added to heavywalled NMR tubes and degassed by freeze−thaw−pump cycles until no more gas bubbles formed upon thawing. The tubes were then flame sealed and wrapped in Al foil to avoid light exposure. The flame-sealed tubes were heated between 100 and 220°C in 10°C increments and monitored by 1 H NMR experiments on every hour of heating if not otherwise stated.
X-ray Powder Diffraction. The X-ray powder diffractograms were obtained using a Malvern Panalytical Empyrean diffractometer in a symmetric θ/2θ configuration, operating at a voltage of 45 kV and current of 40 mA, and using a Cu Kα X-ray source (λ = 1.5406 Å). Ni foil was used to filter the Kβ radiation.
Quantum Chemical Computations. All quantum chemical calculations were preformed using Gaussian 16. 50 Geometry optimizations and harmonic normal-mode vibrational calculations were performed using the long-range corrected hybrid DFT method LC-ωHPBE 51,52 and def2TZVP 53,54 basis set. Solvation was accounted for using the SMD continuum solvation model. 55 Minima were confirmed to have no imaginary frequencies. NBO analysis was performed on the minimized structures using NBO 7.1 56 interfaced from Gaussian 16.